The present invention relates to interference reduction in a cellular mobile radiotelephone system and more particularly to reduction of interference in such a system resulting from a conventional, unrestricted directed retry function.
In mobile radiotelephone systems, one of the main technological challenges is to provide a sufficiently large number of radio channels to meet system demand while at the same time minimizing interference of those channels with one another. In cellular mobile radiotelephone systems, this problem has been addressed by allocating frequencies to cells according to a frequency reuse pattern such that a specified distance, known as the frequency reuse distance, separates geographical areas using a common frequency. Such a technique has proven effective in preventing undue interference of same-frequency channels.
Interference can also occur, however, between different frequency channels assigned to adjacent cells. Normally, transmission power on cell transmitters is regulated so as to minimize such interference. If, however, a mobile station transmitting on the frequency of one of two adjacent cells is located relatively far inside the interior of another of the two adjacent cells, the mobile station becomes a local interference radiator, and interference may become quite pronounced.
Such a situation may arise, for example, when a call access is attempted at a time when all of a cell's voice channels are occupied (cell congestion). The switching center typically responds to such a situation by sending a "directed retry" instruction to the mobile station attempting access together with a list of the frequencies of all of the congested cell's neighboring cells. The mobile station then monitors reception on each of the neighboring cell's frequencies and chooses a strongest received frequency on which to reattempt access. Given a flat topography, the frequency chosen will usually be that of the neighboring cell closest to the mobile station. Because of varying topography, however, the propagation paths and resulting signal strengths may also vary such that the strongest received channel may not always be that of the closest neighboring cell but rather may be that of a more distant neighboring cell. This increases the potential for audible interference.